1. Field of the Invention
The present invention relates to a feed-forward distortion compensating system in optical communication, and more particularly, to a feed-forward distortion compensating system which can restrain modulation distortion of a light output by feed-forward control, in a light transmission unit which is used for analog light transmission such as light SCM transmission in CATV and so on.
2. Description of the Related Art
FIG. 1 is a block diagram illustrating the structure of a light transmission unit to which a conventional feed-forward distortion compensating system is applied. Referring to FIG. 1, the conventional feed-forward distortion compensating system is composed of a signal generator 1, a first laser diode 2, a splitter 3, a photoelectric converter 4 of a PIN diode and so on, an inversion type amplifier 5, a distortion signal detector 6, a second laser diode 7, a synthesizing unit 8, a first delay section 9, and a second delay section 10. In this case, thick lines indicate optical fibers and thin lines indicate coaxial cables.
The signal generator 1 outputs an analog electric carrier signal a in which a plurality of electric signals have been multiplexed in frequency. The analog carrier signal a is branched into a carrier signal a' and a carrier signal a". One of the branched carrier signals, a', is supplied to the first laser diode 2. The first laser diode 2 modulates the branched carrier signal a' from the signal generator 1 in light intensity to emit a main light signal b. As a result, the main light signal b contains modulation distortion together with the carrier signal. The main light signal b which has been outputted from the first laser diode 2 is branched into two parts, i.e., light signals c and d by the splitter 3.
The light signal c from the splitter 3 is supplied to the photoelectric converter 4. The photoelectric converter 4 converts the light signal c into an electric signal e. The electric signal e is supplied to the distortion signal detector 6 as an electric signal f after being amplified by the inversion type amplifier 5. The branched carrier signal a" is supplied to the distortion signal detector 6 after being delayed by a predetermined time period by the delay line 9 to match to the electric signal f in synthesization timing. The distortion signal detector 6 synthesizes the electric signal f from the inversion type amplifier 5 and the delayed carrier signal a".
At this time, because the phase of the electric signal f from the inversion type amplifier 5 and the phase of the delayed carrier signal a" are opposite to each other, the carrier components of these signals are canceled. Therefore, the electric signal g which is outputted from the distortion signal detector 6 contains only a distortion component which has been generated in the first laser diode 2.
The distortion component g which has been detected by the distortion signal detector 6 is supplied to the second laser diode 7. The second laser diode 7 modulates the distortion signal g from the distortion signal detector 6 in light intensity and generates an auxiliary light signal h.
The auxiliary light signal h from the second laser diode 7 is supplied to the synthesizing unit 8. The synthesizing unit 8 synthesizes the main light signal d from the first laser diode 2, i.e., the other of the light signals branched by the splitter 3 and the auxiliary light signal h from the second laser diode 7. In this case, the main light signal d is supplied to the synthesizing unit 8 after being delayed by a predetermined time period by the delay line 10 to match to the auxiliary light signal h in synthesization timing.
At this time, the phase of the main light signal generated in the first laser diode 2 and delayed by the delay line 10 is opposite to the phase of the auxiliary light signal h generated by the second laser diode 7. Therefore, a distortion component is substantially removed in the signal i which is output from the synthesizing unit 8 such that the signal is a low distortion signal.
Such a feed-forward distortion compensating system is disclosed in Japanese Laid Open Patent Disclosure (JP-A-Heisei 2-143732).
However, in the above-mentioned conventional feed-forward distortion compensating system, the amplitude characteristic and phase characteristic of the distortion signal generated in the second laser diode 7 is not always coincident with those of the distortion signal generated in the first laser diode 2. Therefore, it is difficult to achieve a large distortion improving quantity.